Glad to hear that the repair went well ("fingers crossed" :). I'm doubtful about the long-term effectiveness of JB Weld when subjected to the stress and vibration of planing.
I recently experienced a similar AP12 breakdown and thought it might be helpful to the AP12 community to describe it here. My failure, like Ron B's, was related to a jam-up of the feed roller drive train. In my case, the jam-up was clearly caused by planer chips packing into the drive chain recess in the side of the planer frame casting. There apparently was a build-up of chips between the drive chains and the outfeed roller drive sprockets which caused the chains to ride up on the sprockets and jam against the side of the drive chain recess. The drive chains were so tightly stretched around the sprockets by this build-up of chips that I had to snip the chains with a small bolt cutter in order to disassemble the chain drive. Of course the jam-up resulted in several sorts of mahem in the feed roller drive mechanism. In my case, there was no damage to the drive sprockets. The gearbox casting broke in the way Ron describes. Also, when I got inside the gearbox I discovered that all the teeth had been sheared off of the small pinion gear which sits in the middle of the gear drive train.
While an argument could be made that the best course of action would have been to throw the planer in the dump and buy a new one, I decided to see if I could fix it. I was able to buy all the repair parts I needed from ToolPartsDirect.com, (a really good source for repair parts for all sorts of tools and machinery) for about $100. This included a new gearbox housing, bearings, bushings, gears, chains, shafts and keys. I did not cut corners here as the disassembly/reassembly process is time-consuming and I wanted to end up with a reliable piece of equipment.
I am presently waiting for the replacement parts to arrive. In the meantime, while the machine is disassembled, I have decided to cut inspection ports in the steel sides which support and guide the cutterhead so that I will be able to monitor chip buildup and easily blow chips out of the feed roller drive system before they cause problems. I cut these ports or windows by drilling holes at the four corners of each port and then saber sawing between them. They measure about 5 x 4 inches and are covered by pieces of clear lexan attached with small machine screws in each corner.
I see that Ryobi has replaced the AP12 with a redesigned 13" planer which may avoid the problem of chip build-up in the inaccessible feed roller drive system.
Cle Elum, WA